Hey! You only quoted part of the post - omitting the important bits about non-rigidity and elastic coupling.
You’re right, I apologize, I did not re-read what you wrote in post #40 carefully enough.
You are quite right. I overstated. It is only silly if we are talking about the contact face of things moving toward or away from one another.
You are obviously more easily pleased than I am. There have been many explanations of what is happening and the ways that calculations could be made but the central issue of stationary/non-stationary at the same time remain.
I like this answer. The problem is not markedly different but conceptually it doesn’t seem so difficult. It’s true I’m hung up on the zero point. I accept it doesn’t have any particular significance. I mught even be happy! Thanks.
Yes, it has. It was explained at posts 13, 17, 18, , 37, 54, 63, and not to mention most of the posts in the entire thread on the topic linked at post 23. The answer is simply flex. And when you say:
the gulf between reality and your understanding becomes fully apparent. In fact, precisely this occurs all the time because there is a slight slack in the coupling of train carriages. When the engine starts moving at the front it can be a good few seconds before the carriages at the back start. When you grok this you may have a chance of understanding the answers you are being given to your question.
And frankly unless you can explain precisely what it is about the trampoline or rubber sheet analogies you have been given that does not answer the “central issue” you are querying, I don’t see much point in continuing to try to explain.
You are wrong. The analogy doesn’t hold up. The energy balance simply doesn’t pan out.
Let me try another tack with you. Consider a system composed of two atoms, one Iron (Fe) and one Carbon (C). Fe is moving at +100km/h along an axis. C is moving at -1km/h along the same axis. They “collide inelastically” as you propose. The Fe atom is moving faster and is heavier than the C atom, therefore it has more energy. Then how can the C atom stop the Fe atom when reversing direction? Where does the missing energy come from?
When you consider the case of a metal sheet against a bee ball it only makes this imbalance even more extreme.
It won’t. The flex will occur within the inter atomic forces and the Fe atom will not stop. This is not incompatible with my position.
Why so? All that is required is for there to be sufficient energy in the bee ball to balance off against so much of the metal sheet as is flexing and the extent of its flex.
I don’t think our positions are actually in disagreement. You are just considering examples at the atomic level where the “flex” (in the form of inter atomic forces) can occur without stopping the atom as a whole, and I am considering a more general proposition.
In the example of a baseball hitting a train with a trampoline on the front, what do you think the patch of the trampoline in contact with the most trainwards part of the baseball would be doing at the moment when the baseball is turned around and reverses direction? What speed would it be going?
Glad to see not everyone has abandoned this thread.
I wouldn’t call what I think you are describing as flex, since that’s a macroscopic concept, but so far so good.
But there isn’t sufficient energy to stop the metal contact patch, that’s my point exactly.
If it doesn’t stop the atoms, it doesn’t stop the body composed of those same atoms. The bee compresses the metal. It doesn’t necessarily stop it at any point. Another analogy:
Consider two magnets, with the same geometry, oriented to oppose each other, moving along a symmetry axis on a collision course. Magnet A is twice as massive and has twice the speed of magnet B. They oppose each other with equal in magnitude but opposite in sign forces. This force acts at a distance. Magnet B will stop and reverse direction, magnet A will only slow down, never actually stopping during the collision, which happens over a finite time and distance.
Magnet A is the train contact patch and magnet B is the bee. I hope this analogy is clear, a diagram would help here but unfortunately I flunked my Physics ASCII drawing class.
I’m certainly not claiming what you suggest can’t happen. The baseball in all likelihood will stop the trampoline at the contact point. But I don’t think the analogy holds for the bee-train collision.
So we both accept that if the small thing is hard and heavy enough, and the big thing is soft and light enough, the latter will flex and a point on it will stop. I accept that if the small thing is soft and/or light enough and the big thing is heavy and hard enough, no part of the big thing will stop as the “flex” could be at a sub-atomic level.
The difference between us is that I don’t know which case the bee hitting the train will fall into, and you think you do know that but have presented no quantified data that would enable you to reach the conclusion you have.
There is no “flex” at the sub-atomic level. And there are still some pretty wide gaps in your review, regarding conservation of momentum.
Now you’re just backpedaling and being disingenuous.
You’re just quibbling with terminology. I put “flex” in quotes. I know it’s not flex in any conventional sense but it is flex in the sense of something that allows two things to affect one another in a non-rigid way. As to the alleged gaps regarding conservation of momentum, you would have to say where you perceive them to be before I could comment. Until then, you are just indulging in reflexive naysaying.
Nonsense. You have said, not two posts ago, that in a particular small hard object, large flexy object collision, a part of the latter would stop. You have not supplied any data as to where the line would be between that and a collision where the latter would not stop, and everything would occur at the sub-atomic level. Yet you have stated categorically that the bee/train collision would occur via the latter. Provide some data or a cite, or accept you are speculating.
You need a better word then.
I meant that you did not account for the kinetic energy of the bodies involved. Even a very light body (relatively to another) travelling very fast can carry a lot of energy, as I’m sure you know.
I think I already laid out my arguments on this quite clearly. For instance, the magnet analogy and the atomic collision example.
You agreed the Fe atom would not stop. I don’t see any reason to think a fast moving, massive solid network of Fe atoms and a small, slow carbon ball would behave any differently. Maybe you can shed some light on this?
Beyond that, I don’t think I can be any clearer . The burden is on you to prove me wrong. Feel free to disagree, it was still a fruitful discussion for me.
It is also possible to have a situation where a trampoline colliding with a baseball ball would not actually stop, although the speed differential would have to be huge and the trampoline surface would stretch and slow down immensely, but never actually stopping. Of course, in a realistic scenario the trampoline fabric would tear much sooner than this.
I know it’s not “cites and hard” data but there you go.
Or better said, this scenario is completely unrealistic because a real trampoline could never attain the speeds required for this, and even if it could the baseball would probably explode upon impact.
Back of the envelope calculations say the trampoline speed would have to be approximately 100 times the speed of the baseball. It’s not as much as I thought…
You’ll have to explain what you see as the problem more coherently. What aspect of kinetic energy do you see as unaccounted for? Frankly, given that you have already accepted that at a sufficiently macro scale, and with a sufficient degree of elasticity involved, the contact area of the larger object will stop (ie we’re in agreement about this) why are you continuing to suggest there is a problem with the idea?
You did and I accept them. The only point I make is that if we are in agreement that at what we might call the extreme micro scale the more massive object would not stop, and if we are in agreement that at the extreme macro/trampoline scale it would, the only thing in issue is where the dividing line lies.
Without quantified data, you simply can’t say. Which is why I don’t express a view on where the bee/train example lies. You do, so onus of proof is on you.
Sure thing. When one atom collides with one other, (and assuming there is no chemical or nuclear reaction taking place) each object is indivisible, and the elasticity in the collision can only be through the interaction of fields. So the calculations are simple: one part of the Fe atom cannot be moving at a different speed to another part of it.
Contrastingly, once we get to objects made of multiple atoms like, say, a train, one part of it can be moving at one speed and another part can move at different speed, while they are attached to one another, because materials can be flexible. Ain’t no such thing as a “solid” network of Fe atoms, in any absolute sense.
You did, here:
And again here:
Moving on:
Of course. But seeing as every Fe atom pushes or collides with the surrounding outermost atoms, I maintain there is not enough energy in the bee to stop the train surface, relative to an external observer. Instead there will be a compression wave squashing the bee.
You’re right, I see. I accept your correction that I adjusted my position to be a synthesis of both our positions.
However, the fact is that right now you are saying that it is one thing and not the other, and you have no quantified data.
ISTM that it may be that as we move more and more towards the macro scale, the contact point on the larger object gets closer and closer to being stopped, but whether the surface actually stops depends on the atomic makeup of each object.
Or could it perhaps depend upon the harmonics of the thing struck? When the bee hits it’s going to cause the front of the train to reverberate. What is going to affect the speed of vibration? Is that speed going to be higher or lower than the speed of the train?